The present invention generally relates to the control of high-speed sheetmaking machines.
One shortcoming of conventional systems for controlling the operation of high-speed sheetmaking machines is that the response times of the systems are relatively slow following abrupt changes in process conditions such as caused by sheet breaks, reel or grade changes, or during start-up. The slow responses of the control systems assure control stability under normal operating conditions but may allow substantial quantities of substandard sheet material to be produced before effective corrective actions are implemented following abrupt process changes.
In the sheetmaking art, it is well known to use the Smith Predictor control scheme. This control scheme is described in an article entitled "A Simple Adaptive Smith-Predictor for Controlling Time-Delay Systems," T. Bahill, IEEE Control Systems Magazine, Vol. 3, No. 2 (May, 1983) pp. 16-22. The Smith Predictor control scheme is often employed in time delay compensation control systems, which is to say in systems having "dead times." Accordingly, the Smith-Predictor control scheme is appropriate for use in sheetmaking systems because there are unavoidable delays between the times at which sheets are acted upon by manufacturing equipment and the times at which sheet properties are measured.
In control systems incorporating the Smith Predictor control scheme, errors in estimating the gains and time constants of the systems can cause control instabilities. Thus, in conventional practice, controllers in systems based upon the Smith Predictor control scheme are tuned to achieve long-term stability at the expense of rapid responses to transient conditions. Another way of stating this is to say that, conventionally, controller parameters in Smith Predictor systems are selected so that the systems respond relatively slowly, but stably, to detected variations in a property being controlled.
In view of the foregoing discussion, it can be appreciated that there is a need for control systems that rapidly adjust sheetmaking systems when process conditions change abruptly but, under normal conditions, provide smooth and stable operation.